The invention is in the field of electronic reproduction technology and is directed to a method for the operation of an engraving element in an electronic engraving machine for engraving printing forms, particularly printing cylinders, for rotogravure.
In an electronic engraving machine, for example, an electromagnetic engraving element having an engraving stylus as a cutting tool moves along a rotating printing cylinder in an axial direction. The engraving stylus controlled by an engraving signal cuts a sequence of depressions, called cups, arranged in a raster into the generated surface of the printing cylinder. The engraving signal is formed from the superimposition of an image signal representing the gradations between xe2x80x9cblackxe2x80x9d and xe2x80x9cwhitexe2x80x9d with a periodic raster signal. Whereas the raster signal effects a vibrating lifting motion of the engraving stylus for generating the raster, the image signalxe2x80x94in conformity with the gradations to be reproducedxe2x80x94controls the depths of the cups engraved into the generated surface of the printing cylinder.
Given an electromagnetic engraving element, the drive system for the engraving stylus is essentially composed of a stationary electromagnet driven with the engraving signal in whose air gap the armature of a rotational system moves. The rotational systemxe2x80x94other than the armaturexe2x80x94comprises an armature shaft, a shaft bearing and a damping mechanism. One end of the armature shaft is designed as a resilient torsion rod clamped stationarily in space, whereas the other end carries a lever-like stylus holder for the engraving stylus.
As a result of the engraving signal, an alternating magnetic field is generated in the electromagnet that exerts electrical torques onto the armature that are opposed by the mechanical torque of the torsion rod. The alternating electrical torques cause a vibratory motion of the armature shaft from the quiescent attitude defined by the torsion rod by angles that are proportional to the amplitudes of the engraving signal. As a result of the vibratory movement of the armature shaft, the stylus holder together with the engraving stylus executes lifting motions directed onto the generated surface of the printing cylinder that define the penetration depths of the engraving stylus into the generated surface of the printing cylinder.
The alternating magnetic field in the electromagnet generates alternating current losses in the armature that are dependent on the frequency of the raster signal. The alternating current losses heat the armature, the armature shaft and the stylus holder together with the engraving stylus slowly from an initial temperature at the start of engraving up to a stable operating temperature during engraving. The heating causes an expansion of the armature shaft and of the stylus holder as well as a modification of the magnetic permeability of the pole shoe iron of the electromagnet and of the armature.
At traditional frequencies of the raster signal (engraving frequencies), the temperature changes between the initial temperature at the start of engraving and the stable operating temperature as well as given interruptions in engraving are so slight that the expansion and permeability changes caused by the heating do not disadvantageously influence the quality of the engraved cups.
In practice, there is a demand for shorter engraving times or, respectively, for higher engraving speeds. In order to achieve these demands, the circumferential speed of the printing cylinder, the axial feed rate of the engraving element and the engraving frequency must be raised.
Since a disprotionately high electrical power is required for operating an engraving element at a higher engraving frequency, greater temperature changes arise due to a higher operating temperature, these potentially leading to inadmissible expansion and permeability changes and, thus, to the engraving of faulty cups.
It is therefore an object of the present invention to improve a method for the operation of an engraving element in an electronic engraving machine for engraving printing forms, particularly printing cylinders, for rotogravure such that disturbing temperature changes are avoided, in order to thereby achieve a good engraving quality.
According to the method of the invention for operation of an engraving element in an electronic machine for engraving a printing cylinder for rotogravure, before a start of engraving and/or during an engraving interruption, heating the engraving element in order to achieve a good engraving quality. An engraving stylus of the engraving element controlled by an engraving signal then engraves a sequence of cups arranged in a raster onto the rotating printing cylinder. The engraving signal is formed of a superimposition of an image signal representing gradations to be engraved with a periodic raster signal for generating the raster. The engraving element executes a feed motion along the printing cylinder proceeding at an axial direction of the printing cylinder for planar engraving of the cups.